Is the z-axis acme lead screw worth it?

Talk about all things CNC
pastprimitive
Posts: 34
Joined: Wed Aug 13, 2014 4:31 pm
Contact:

Is the z-axis acme lead screw worth it?

Post by pastprimitive » Wed Aug 13, 2014 6:34 pm

So I recently dumped my CNC Shark pro plus that I put in $3,000 used including upgrades I did. Had it for a few years, sold it a week ago for $2,000 after discovering shapeoko 2. The CNC Shark Pro Plus is a capable wood working machine, but that is about it. Just way too flexible for getting the tight precisions I wanted to get. It's made from HDPE reenforced with aluminum. It's an okay design, but there is A LOT left to be desired. The shapeoko2 is leaps and bounds ahead in design, etc. Even though my machine had a relatively large format for price, and lead screws on every access.... all of that was sort of pointless because of the flimsy nature of the HDPE even after being reenforced. And to boot the y axis was a single drive with linear bearings on each side. So the gantry had this nasty tendency to hang up on itself if you looked cross ways at the thing.

Anyhow, excited to build my shapeoko when it gets here in 2 days.

I already upgraded to dual 156oz/in NEMA 23's for the Y-axis, and 260oz/in nema 23's for my X and Z axis. Will probably eventually upgrade to the 1000mm maker slides... but honestly I rarely ever used the capacity of my shark.... So I assumed i'd get much better precision on the 500mm set up, and be that much more capable of handling aluminum.

Also upgraded to a 500 watt DC brushless spindle. While I am sure i'll need to be significantly more conservative milling aluminum in my feed and cut rates... That's ok.

I could only manage so many upgrades at once with never having laid my hands on the machine at all.

Anyway, what I am really curious about is if the z-axis acme lead screw is really worth it. I'll be honest while it seems like a speed, and precision upgrade... I'm not seeing how I'll really be adding that much precision over a 1.25mm pitch with 1/8 micro stepping at what I assume is 200 steps per revolution. So step increments of .00078125mm per step. Now I realize it being a non acme threaded lead screw, and just a plain old standard m8 threaded rod that there will be some sinusoidal variations in my travel per x number of steps depending on where my anti backlash nut lands on the threaded rods sinusoidal variations from the standard thread machining process. But when I am talking about milling at most 3/4" materials for anything I would do, and more commonly 1/4" to 1/8" materials.... It just seems like I would gain no real measured precision from upgrading to a acme threaded lead screw.... Speed of course.... but precision....

The other downer I see is that the M8 seems like it provides significantly more mechanical advantage for my Z axis.... I'd need a lot more powerful motor to equal the down force that can be produced via my M8 threaded rod as compared to my acme 3/8" 12 tpi lead screw.... Which seems like a disadvantage for milling relatively harder materials like aluminum, brass etc.

But then again without having the machine in front of me to test things, and poke around this is really all just theory. I've dealt with similar issues on my Solidoodle 3 3d printer... but I had significantly more Z travel, and filament squashing was very much affected by the sinusoidal variations of the threading on a standard machined rod. Anyhow, would love to hear what I am missing, or if indeed upgrading to an acme lead screw is worth it.

I see the $65 kit here, and part of me says I can buy a 3' bar of the same lead screw off ebay for $20 including shipping plus $10 - $20 for bearings, etc... I've got a lathe to turn the thing down to fit into the bearings... but maybe the lead screws offered on Shapeoko.com achieve better runout tolerances than what I can on my lathe. But as I type this I think.... $25 savings is not worth the headache of spending an hour machining the raw lead screw.

Apologize for the long windedness. Thanks for any input. Feel free to be merciless.
ShapeOko 2 6095. Double Y-Axis Nema 23s (140oz/in), X & Z Nema 23 (262oz/in), TinyG, 500W DC Brushless Spindle w/ 48VDC 10A Supply, Acme Lead Screw, 24VDC 10A Supply, Threaded Inserts Table, Auto-tool height setter.

Auarhau
Posts: 243
Joined: Tue Feb 25, 2014 8:46 pm

Re: Is the z-axis acme lead screw worth it?

Post by Auarhau » Wed Aug 13, 2014 8:20 pm

I've only used it briefly, and I did/do have some problems getting it to run correctly. I was however able to increase my Z seek speeds from 200 mm/min on the m8 rod to 2000-2500 mm/min with the acme screw. The stock screw is plenty accurate and the feed speeds it provides is plenty for everything I've cut so far. The seek speeds though are slow on the stock screw, and that adds up quickly in jobs with lots of Z movements.

Try the stock machine first before you spend a lot of cash on upgrades. The Z acme screw in particular is very quick to install at a later time, so no reason to get it before you need it imo..

What I would try though is getting a longer m8 rod and making a supported bearing on the bottom of the Z, like the upgraded one have. That should be super cheap, and help with aligning the m8 screw (I could never get it to run w/o wobble).

Then get or make the upgrade if you need it.
ShapeOko 2. Nema 17 74 oz·in. GAUPS shield on Arduino Uno. DRV8825 Drivers x4 . Kress 1050 FME-1. Z Acme Screw. Threaded inserts table.

cvoinescu
Posts: 4442
Joined: Thu Jul 19, 2012 6:50 pm
Location: Camberley, UK
Contact:

Re: Is the z-axis acme lead screw worth it?

Post by cvoinescu » Wed Aug 13, 2014 8:34 pm

The threaded rod Z axis is a bit difficult to assemble, because it can be hard to get the rod perpendicular to the bearing when the nuts are tight. Once you've done that, it works fine, although it may still wobble a little.

The Acme screw, even though it has a smaller mechanical advantage, works better, because it has much less friction, and it's very straigt. So you get away with a smaller motor on the Z axis (by the way, yours is way overkill, to the point that its weight is detrimental, in my opinion). Most of the time, you don't need a lot of force on the Z axis, because most milling is sideways. When you plunge straight down, you do it slowly -- forcing the endmill down would be counterproductive (the machine flexes too much), so you don't need a lot of mechanical advantage on the Z axis anyway.

Other advantages of the screw: the threads in the nut are much sturdier, and the bearings holding the screw are preloaded, so they introduce less backlash than the single bearing of the stock Z axis.

Depending on how you cut and on how your CAM optimizes the paths, being able to rapid the Z five times as fast may make some jobs significantly shorter. Even if all you cut are profiles and pockets, it makes a difference for the jobs where there are a lot of them.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

pastprimitive
Posts: 34
Joined: Wed Aug 13, 2014 4:31 pm
Contact:

Re: Is the z-axis acme lead screw worth it?

Post by pastprimitive » Thu Aug 14, 2014 1:28 am

Normally i'd be all about the wait till i've built it stock mind set. But I am rich on cash right now, and don't mind upgrading to something that I will at the very least not be detrimental to my machine. Some "upgrades" I see recommended sometimes are not necessarily an upgrade. And both of you seem to be saying the same thing.

The plunge force being plenty sufficient (especially with the might of a 269 oz/in stepper behind it, especially given the reduced friction from a silky smooth acme lead screw, and the increased job time is a major bonus. Especially for something like milling PCB's where there would be lots of z movement on complicated boards.

Random side note along the lines of overkill I already planned for a serious power supply, and am using tinyG for my electronics setup.

So thanks for helping me decide, and I just ordered it while writing this:D

My gold with the machine was to purchase all the upgrades right now that would either be relatively cheap, and or save me money by not buying doubles of things later on, e.g. two sets of steppers, too small power supply.

I think with my setup I am very well over prepared for upgrading to the 1000mm maker slides if I want, and possibly a 1.5kw spindle if I feel like the 500 watt brushless DC isn't enough.

Interestingly enough I have a LittleMachineSHop.com siege sx2 mini-mill. They use a 500 watt brushless DC motor on the thing and it's honestly super beefy for the size of mill. I can take 1/16" to 1/8" passes with a 1/4" end mill out of aluminum. Although they have geared it down to max out at 2500 rpm via belting... So I realize my 500 watt direct drive spindle will only have about a 1/4 the cutting capacity or less.

but i'm positive for wood it will be plenty sufficient. Especially considering from the sound of it this systems major limits are not from cutting torque, but more from rigidity of the maker slides, and delrin wheels.
ShapeOko 2 6095. Double Y-Axis Nema 23s (140oz/in), X & Z Nema 23 (262oz/in), TinyG, 500W DC Brushless Spindle w/ 48VDC 10A Supply, Acme Lead Screw, 24VDC 10A Supply, Threaded Inserts Table, Auto-tool height setter.

calica
Posts: 107
Joined: Wed Apr 11, 2012 8:13 am

Re: Is the z-axis acme lead screw worth it?

Post by calica » Sat Aug 16, 2014 7:39 am

Could you post a link to your motor? I'm looking to upgrade my dremel and could use recommendations/inspiration. Thanks.

pastprimitive
Posts: 34
Joined: Wed Aug 13, 2014 4:31 pm
Contact:

Re: Is the z-axis acme lead screw worth it?

Post by pastprimitive » Mon Aug 18, 2014 3:24 pm

Yeah, for sure. Although I had to ask for it from the seller because he didn't have it posted on ebay anywhere. So essentially I bought the motor, than asked for it and he told me to just add $25 via his paypal to upgrade to the brushless version.

http://www.ebay.com/itm/151376289717?_t ... EBIDX%3AIT

Anyway, there you go.
ShapeOko 2 6095. Double Y-Axis Nema 23s (140oz/in), X & Z Nema 23 (262oz/in), TinyG, 500W DC Brushless Spindle w/ 48VDC 10A Supply, Acme Lead Screw, 24VDC 10A Supply, Threaded Inserts Table, Auto-tool height setter.

tra
Posts: 63
Joined: Tue Nov 05, 2013 1:27 am

Re: Is the z-axis acme lead screw worth it?

Post by tra » Thu Aug 21, 2014 1:28 pm

I bought the acme upgrade and the performance is worse than the stock M8 threaded rod. I have a LOT of backlash issues. You can actually jiggle whole z-axis up and down and the motion is between the lead nut and the acme screw. It was a waste of $65 for me...not happy with it.

jbrewton
Posts: 40
Joined: Mon May 19, 2014 12:49 pm

Re: Is the z-axis acme lead screw worth it?

Post by jbrewton » Thu Aug 21, 2014 2:35 pm

My only issue was a little movement up and down, but the small spacer provided with the kit took care of that. It moves a lot smoother than the m8.


Sent from my iPad using Tapatalk
Shapeoko 2 Full Kit
Upgrades:
Makita RT0701C

tra
Posts: 63
Joined: Tue Nov 05, 2013 1:27 am

Re: Is the z-axis acme lead screw worth it?

Post by tra » Thu Aug 21, 2014 2:44 pm

I got the screw itself stable, but still have movement between the nut and the screw. I'm working on an anti-backlash solution, but didn't know before buying that the "upgrade" was more trouble than stock.

cvoinescu
Posts: 4442
Joined: Thu Jul 19, 2012 6:50 pm
Location: Camberley, UK
Contact:

Re: Is the z-axis acme lead screw worth it?

Post by cvoinescu » Thu Aug 21, 2014 4:57 pm

tra wrote:I bought the acme upgrade and the performance is worse than the stock M8 threaded rod. I have a LOT of backlash issues. You can actually jiggle whole z-axis up and down and the motion is between the lead nut and the acme screw. It was a waste of $65 for me...not happy with it.
I seem to remember that Improbable Construct offered to replace your lead nut. What came of that?
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

Post Reply